I have the code below that I pluged from SO. My problem is that although I sort of understand how with rewrite True and rewrite False it is suppressed that always the first element of the tuple stays as-is; I cannot find any way to apply this to the last element also. In essence there are times when I would need to change all elements of the tuple but the first and at other times I would need to change all elements except the first and the last. I don't quite understand (yet) how I could do that. Is there any way to match for a specific element eg. the third from the right or so? --Joerg {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} import Data.Typeable import GHC.Generics rewrite_ :: (Generic a, Generic b, Rewriter (Rep a), Rewrite (Rep a) ~ Rep b) => a -> b rewrite_ = to . rewrite False . from class Rewriter f where type Rewrite f :: * -> * rewrite :: Bool -> f a -> (Rewrite f) a instance Rewriter f => Rewriter (M1 i c f) where type Rewrite (M1 i c f) = M1 i c (Rewrite f) rewrite x = M1 . rewrite x . unM1 instance Typeable c => Rewriter (K1 i c) where type Rewrite (K1 i c) = K1 i String rewrite False (K1 x) | Just val <- cast x = K1 val rewrite _ _ = K1 "NIL" instance (Rewriter a, Rewriter b) => Rewriter (a :*: b) where type Rewrite (a :*: b) = Rewrite a :*: Rewrite b rewrite x (a :*: b) = rewrite x a :*: rewrite True b y0 :: (String, Int, Double) y0 = ("something", 3, 4.788) y1 :: (String, String, String, (Int, Int)) y1 = ("something else", "Hello", "NIL", (4,6)) main :: IO () main = do print (rewrite_ y0 :: (String, String, String)) print (rewrite_ y1 :: (String, String, String, String))